The mechanical response of liquid nitrogen-quenched 9.4 Mg-PSZ in which the orthorhombic (o) phase is the major constituent (46 wt%) was investigated using in-situ neutron diffraction during uniaxial compression. The material remains elastic below 1 GPa with a Young's modulus of~242 GPa, second highest of all zirconia-based materials and highest of all zirconia-based ceramics. Beyond 1 GPa, the material develops small plastic strains in a time-dependent manner (i.e., by room temperature creep) although the strains were generally much smaller than the unquenched material, which contains no o phase. As for standard Mg-PSZ, the creep was accompanied by a volume change usually indicative of tetragonal to monoclinic (m) phase transformation; however, the amount of m phase apparent in the neutron diffraction patterns increased only marginally. The magnitude of the volume increase could not be accounted for by the observed increase in the m phase and hence, microcracking is believed to be responsible for most of the volume change. There is some evidence for a small amount of o to m transformation at the detection limit of the phase analysis technique.